"Harmony is a combination of units which are similar in one or more respects.
Harmony is a medium interval or difference in one or more dimensions. Units are harmonious when one or more of their elements or qualities, such as shape, size, or color, are alike."
Take these two circles for example. They are identical in size and shape but different in color. They harmonize insofar as they have similar elements.
"Complete repetition is one extreme. Here, all the units are identical in size, shape, color, value. The effect of complete repetition is monotony.
Discord is the opposite extreme. Discord is a combination of totally unrelated units. The effect of total dissimilarity is discord.
Good design runs the gamut from one extreme to the other.
Harmony is between the two extremes and combines the character of both.
So somewhere between extreme monotony and extreme discord is visual harmony.
"Repetiton, harmony, or discord, therefore, is simply a matter of degree of interval or difference between units. If two units have no dimension or quality that they share in common, they are totally unrelated and represent maximum opposition or contrast. If one of their dimensions is similar or identical, the units are harmonious. If two, more harmonious. If all their dimensions are the same, the units are identical.
These three fundamental forms, repetition, harmony, and discord, and their combinations are the basis of all art structure."